1. Field of the Invention
The present invention relates to electrical circuits and signals, and, more particularly, to an apparatus for regulating electrical voltages and recovering energy normally lost during the termination of electrical signals to a voltage between power supply voltage planes.
2. Description of the Prior Art
Electrical signals travel between generating points and terminating points, or output points and input points. Where only the voltage component of a signal is desired at a terminating point, any inherent signal energy must be dissipated or transferred. Large synchronous systems commonly require numerous instances where clock and data buses are distributed using differential voltage pairs, as well as numerous instances where electrical signals must be terminated to a particular voltage. Such systems typically connect termination resistors to a common, regulated termination voltage plane which has a voltage between, or intermediate to, the upper and lower power supply voltage planes. The regulatory requirements for a termination voltage plane are unique in that the termination resistors become, in effect, a current source that the regulator must sink to circuit ground. Thus, such systems must simultaneously regulate voltage planes, sink current produced by termination resistors, and absorb power as well. Common commercially available regulator modules and integrated circuit regulators are not suitable for this application, being designed only to source current and deliver power to a load.
Current mode switching logic systems, including emitter-coupled logic (ECL) and positive emitter-coupled logic (PECL) systems, require strict regulation of termination voltage planes in order to maintain the voltage difference necessary to prevent the bipolar junction transistors from entering saturation. Furthermore, large high-performance ECL and PECL systems can generate a substantial amount of signal termination energy. Although ECL is the fastest logic family, it can be so wasteful of energy as to make ECL undesirable for many systems which would otherwise benefit from its speed advantage.